(1) Field of the Invention
The invention relates to a metallization process for integrated circuit devices, and more particularly, to a metallization process with good step coverage for high aspect ratio, submicron contact/via holes while maintaining an effective alignment cross-mark structure.
(2) Description of the Prior Art
Typically, step coverage is poor for the metallization of high aspect ratio, submicron contact or via openings. Typical solutions use a plug process and planarization to completely fill the contact or via holes. Such processes are described in "Development of a Planarized Al--Si Contact Filling Technology" by Hisako Ono, Yukihiro Ushiku, and Takashi Yoda, IEEE VMIC Conference Proceedings, Jun. 12-13, 1990, pp. 76-82, "Planarized Aluminum Metallization for Sub-0.5 um CMOS Technology" by F. S. Chen, Y. S. Lin, G. A. Dixit, R. Sundaresan, C. C. Wei, and F. T. Liou, IEDM 90, pp. 51-53, "Al-PLAPH (Aluminum-Planarization by Post-Heating) Process For Planarized Double Metal CMOS Applications" by C. S. Park, S. I. Lee, J. H. Park, J. H. Sohn, D. Chin, and J. G. Lee, IEEE VMIC Conference Proceedings, Jun. 12-13, 1990, pp. 326-328, and U.S. Pat. No. 4,970,176 to Tracy et al.
In their paper, Ono et al describe a plug process in which a barrier layer is sputter deposited, followed by an Al--1% Si film at 500.degree. C., which is then etched away. An Al-Si-Cu film is then deposited, patterned, and etched to form a plug for the contact openings. Chen et al, in their paper, describe a process similar to Ono's, but without the extra process steps of removing the first aluminum film and depositing a second aluminum film. Park et al describe a process of sputtering Al--1% Si--0.5% Cu at below 100.degree. C., heating to 400-550.degree. C. to anneal, then depositing additional aluminum to the desired thickness. These plug methods all require the contact holes to be completely filled.
The patent to Tracy et al describes a process consisting of depositing an aluminum alloy at a temperature of less than or equal to 200.degree. C. This deposition will comprise 50 to 75% of the total thickness required to fill the contact hole. The temperature is increased to 400.degree.-500.degree. C. to reflow the aluminum, then more is added at one or more rates of sputtering. The problem that may occur here is that the thickness of the "cold" aluminum alloy may cause voids within the contact holes which may not be filled by the subsequent reflow.
A further consideration in contact hole metallization not evident in the references cited above is the issue of lithographic alignment marks. Alignment marks are formed as holes in the dicing lines between devices so as to properly align the photomask and the wafer. If the alignment holes are filled with metal, effective detection becomes difficult or impossible. In his U.S. Pat. No. 5,002,902, Watanabe discusses this problem. He says that it is common to provide additional alignment marks after any sputtering. This requires additional steps and must be done each time a conductor layer is provided. Watanabe describes a method of forming alignment holes deeper than the contact holes so that the alignment holes will not be filled when the contact holes are filled. U.S. Pat. No. 4,981,529 to Tsujita describes how to position alignment marks so that they can be recognized properly under a resist film.